1. Field of the Invention
The present invention relates to ultrafine metal oxide particle dispersion liquids in which ultrafine metal oxide particles are dispersed and ultrafine metal oxide particle thin films composed of metal oxide nanoparticles and having excellent dielectric properties.
2. Description of the Related Art
In recent years, higher-performance devices have been intensively researched and developed with the increasing need for the miniaturization of devices. For example, composite metal oxides such as barium titanate and lead titanate zirconate are widely used for devices such as monolithic capacitors and actuators since such materials have excellent dielectric and piezoelectric properties. Development of smaller, higher-performance devices demands the establishment of techniques for manufacturing thinner, higher-quality films composed of finer particles.
One of such thin-film manufacturing techniques is the preparation of a coating liquid for thin films by a microemulsion method or metal organic decomposition process (MOD). The microemulsion method is schematically illustrated in FIG. 1.
FIG. 1 shows a schematic illustration of a microemulsion and enlarged sectional views (half) of a droplet 1 and its vicinity contained in the microemulsion before and after hydrolysis performed by adding a composite metal alkoxide. FIG. 1 shows the droplets 1 (in the microemulsion), a surfactant 2, a cosurfactant 3, water 4, a reaction product 5, and a dispersion medium 6, such as cyclohexane.
The microemulsion method is performed by adding water to a hydrophobic medium together with a surfactant to prepare a microemulsion in which fine water droplets (the droplets 1 in FIG. 1) are dispersed and then introducing and reacting a starting material in the water droplets through, for example, hydrolysis to produce crystallized ultrafine metal oxide particles having a small particle size distribution. A transparent dispersion liquid in which the resultant ultrafine metal oxide particles (also called metal oxide nanoparticles) are well dispersed and do not aggregate can be prepared by adjusting the content of water in the microemulsion to the minimum amount of water required for the hydrolysis.
This transparent dispersion liquid may be deposited by, for example, spin coating to form a thin film composed of fine nanoparticles having a size of about 20 nm.
On the other hand, MOD is a method for forming a thin film by applying (for example, by spin coating), drying, and firing a MOD coating liquid prepared by dissolving a metal alkoxide or carboxylate in, for example, an aromatic solvent.
Japanese Unexamined Patent Application Publication No. 10-87329, for example, discloses a coating liquid containing organic metal compounds produced by, for example, reacting an alkoxide or complex of a specific metal with an carboxylic anhydride, glycol, a β-diketone, or a dicarboxylic monoester. The organic metal compounds have high solubility to organic solvents, and the coating liquid has high storage stability. In addition, the coating liquid may be partially hydrolyzed for use as a sol-gel coating liquid to reduce the content of organic components in the coating liquid.
For example, a barium titanate (BaTiO3) nanoparticle thin film formed with a transparent barium titanate nanoparticle dispersion liquid prepared by the microemulsion method with the minimum amount of water required for the hydrolysis has high crystallinity and excellent dielectric properties though the film is composed of fine particles having a size of 20 nm or less. This is probably because a gradual reaction does not allow the particles to take in excess water. In addition, this dispersion liquid, having high dispersibility and transparency, enables the production of an excellent thin film having uniformity and small surface roughness.
A coating liquid prepared by a conventional microemulsion method, however, undesirably requires the repetition of coating many times to form a thin film having a desired thickness because the concentration of the coating liquid is low and therefore the rate of deposition is low, namely 10 to 20 nm for each coating.
A higher rate of deposition for each coating is possible by the following approaches:
(1) The hydrophobic medium in the microemulsion is removed to condense the microemulsion, thereby increasing the concentration of the composite metal nanoparticles produced.
(2) The content of water in the microemulsion and the amount of metal alkoxide added, which corresponds to the content of water, are increased to increase the concentration of the composite metal nanoparticles.
(3) The amount of metal alkoxide added is increased with the composition of the microemulsion unchanged to increase the amount of residual metal alkoxide relative to that of the composite metal nanoparticles and increase the concentration of the composite metal nanoparticles.
According to the first approach, the removal of the hydrophobic medium increases the proportion of the residual surfactant, thus undesirably increasing the viscosity of the dispersion liquid and producing porous films due to loss of the surfactant during firing.
According to the second approach, the content of water for the hydrolysis in the microemulsion is increased to increase the possible amount of metal alkoxide added. In this approach, however, an increased content of water relative to that of metal alkoxide causes aggregation at the beginning of the addition of the metal alkoxide to the microemulsion to produce a cloudy dispersion liquid unless the metal alkoxide is exceptionally rapidly and uniformly added. Such a cloudy dispersion liquid is extremely difficult to disperse again.
According to the third approach, a dispersion liquid prepared by adding an excessive amount of metal alkoxide stock solution with the composition of the microemulsion unchanged contains a larger amount of residual unreacted metal alkoxide. In the microemulsion method, a metal alkoxide, which is readily hydrolyzed, is used because the hydrolysis must be performed with a small amount of water. The residual unreacted metal alkoxide is therefore readily hydrolyzed by moisture in air. As a result, this dispersion liquid undesirably exhibits low storage stability and repeatability.
On the other hand, a stock solution prepared by MOD has about several to tens of times as high a concentration as that prepared by the microemulsion method, thus providing a higher rate of deposition, namely 30 to 80 nm for each coating. In addition, the stock solution prepared by MOD is directly used in principle, thus allowing uniform deposition and less leakage current. Furthermore, MOD can be modified for improving the storage stability and preventing striation, and has also been studied for crystallization with high solubility at relatively low temperatures. MOD, however, has a fundamental problem: this method poses difficulty in providing good electrical properties, though producing an apparently good-quality film that is relatively dense and smooth. This is probably because the resultant film has insufficient crystallinity.